Two medium nectarines can be substituted for the mango.
To prepare the mango, score the skin lengthwise into fourths with a knife, and peel like a banana. Cut peeled mango lengthwise close to both sides of the seed, then dice.
To prevent fish from sticking to the grill, lightly brush the grill rack with vegetable oil. To avoid flare-ups, don't spray cooking spray over hot coals.